Cadmium electroplating and plating baths therefor



United States atent O 2,990,344 CADMIUM ELECTROPLATING AND PIJATING BATHS THEREFOR Peppino N. Vlannes, Fort Washington Forest, Md. (8535 Van Buren Drive, Washington 22, D.C.), and Simon W. Strauss, 308 Connecticut Ave. NW., Washington 8, DC. No Drawing. Filed Dec. 31, 1958, Ser. No. 784,401 'Claims. (Cl. 204-50) (Granted under Title 35, U.S. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates to a process for the electrodeposition of cadmium and to electroplating baths therefor.

Electroplating with cadmium is customarily done from aqueous cyanide bath. While this process has a number of good qualities, it is a well-known source of introduction of hydrogen into metals. Hydrogen is introduced into the metal by reason of the fact that both hydrogen and cadmium are deposited at the surface of the cathode and part of the deposited hydrogen is absorbed by the metal being plated. The presence of hydrogen in the metal reduces the fatigue strength of the metal through hydrogen embrittlement.

In recent years there has been an increasing incidence of service failures in which high strength steel articles which have been cadmium plated by the cyanide bath process have failed by delayed fracture. This phenomenon of delayed fracture, also referred to as static fatigue, is known to be associated with the presence of hydrogen in the steel. Delayed fracture is fracture of the metal after a period of time under an essentially constant stress much lower than the yield strength of the metal.

It is a principal object of the present invention to provide a new process for the electrodeposition of cadmium. It is also an object to provide a process for electroplating cadmium froma non-cyanide bath in which deposition of hydrogen and consequent hydrogen embrittlement of the metal to be plated is markedly reduced over that occurring in the aqueous cyanide bath process. It is a further object to provide new non-cyanide electroplating baths. It is another object to provide a process for preparing the new non-cyanide electroplating baths.

These and other objects can be accomplished in accordance with our invention by electrodepositing cadmium from a bath comprising an aqueous solution of a cadmium salt other than a cyanide to which an amount of triethanolamine has been added which is hereinafter defined below. An excess of triethanolamine may be used in the baths and is desirably present therein. The pH of the baths may range from about 7 up to about 10 and in each instance will depend on the concentration of triethanolamine in the bath.

The preparation of the new electroplating baths of the invention is accomplished by first making up a water solution of the cadmium salt of the desired concentration and then adding the triethanolamine to the aqueous solution of the cadmium salt. On addition of the triethanolamine, a precipitate will occur in the bath which dissolves directly on stirring. If, on the other hand, the reverse order of addition of the components is employed, in which the cadmium salt solution is added to the triethanolamine, a precipitate occurs which dissolves only after several hours of stirring. Suitable cadmium salts for preparation of the baths of the invention are, for example, cadmium sulfate, -chloride, -bromide, -acetate, -tartrate, etc. For practical results in reduction of hydrogen embrittlement in the metal to be plated, the concentration of cadmium in the baths should not be below about 0.5

2,990,344 Patented June 27, 1961 'ice molar. A general range of useful concentrations of cadmium in the baths is from about 0.5 to 2 molar. The molar ratio of triethanolamine to cadmium may be from 1:1 to about 4:1 with a resulting pH in the bath in the range of above 7 up to about 10. The optimum concentration is a molar ratio of these components of from 1.25:1 to 1.55:1 with a resulting pH in the bath in the range of about 7.5 to 9. The required pH of the baths is provided by the triethanolamine. Additional alkalinity provided therein from other sources is not necessary for the production of successful cadmium platings by the process of the invention. In this respect, good platings were not had from baths where caustic soda had been added thereto.

The composition of typical electroplating 'baths in accordance with our invention is contained in the table below in which the amount of the starting components in the solutions is in moles per liter.

Triethanolamine 30dS 04.81120 pH of Solution The electroplating process of the invention, aside from the composition of the plating bath, can be carried out in conventional manner. The plating vessel can be of any suitable design and material. Plating can be done from the static cell or from the barrel or as rotating cathode plating with good results in reduced hydrogen deposition and hydrogen embrittlement of the metal to be plated. The process is applicable to the plating of mild and high strength steels, of iron, and of copper and copper-rich alloys, such as brass, and of other metals which are plated with cadmium. The process may be conducted at temperatures in the bath of from about --l0 C. to the boiling point of the solution. However, it is preferable that the process be operated with the bath at room temperature, or generally in the range of 20 to 25 C. The anodes are of cadmium and may be in the form of balls, strips, liners or rods, etc., connected to a source of current and placed in contact with or suspended in the plating bath. The cathode is connected to a current supply so as to provide a current density for plating which is in the range of from about 4.5 to amperes per square foot of cathode area. In general, for optimum plating conditions, the current density should be approximately 25 amperes per square foot of cathode area. The cathode should be clean and smooth following known good plating practice. For steel, in particular ultra highstrength steel, a pickling or acid cleaning step should be omitted to preclude hydrogen emb-rittlement known to be caused by the acid immersion procedure.

Comparative tests performed with the use of a cylindrical notched bar of AISI 4340 steel of approximately 285,000 p.s.i. ultimate tensile strength as substrate and plating of the bars with cadmium from baths of the composition set forth in the table above and from a standard aqueous cyanide bath demonstrated the marked reduction in hydrogen deposition and hydrogen embrittlemerit to be obtained by electroplating with the process of our invention. The current density was 15, 25, 38 and 51 amperes per square foot for the platings by the process of the invention and 25 amperes per square foot for the platings by the cyanide bath process. The platings by the process of the invention and by the cyanide bath process were performed at room temperature. The plating time in all cases was about 30 minutes. The plating thickness on the barrel of the specimens was about .0005

inch average. The steel bars were cleaned for plating by sandblasting rather than by pickling in order to eliminate the hydrogen embrittlement known to be caused by pickling alone.

The plated steel specimens were evaluated for hydrogen embrittlement by delayed fracture test. The test was that of Raring and Rinebolt, ASTM Bulletin No. 213, pages 74-76, April 1956. The load applied was 75% of the ultimate tensile strength of the unplated bar. This represents very severe test conditions. The specimens plated with cadmium by the cyanide bath process fractured on loading. None of the specimens plated with cadmium by the process of the invention fractured on loading. The average time before fracture for 70% of the specimens plated by the process of the invention was about'11.5 hours, with the remaining 30% of the same being intact for the duration of the test which was 100 hours;

Another comparative test was made by the older tensile testing method which involves application of an increasing load in tension on the specimens. The steel specimens were in all respects the same as those used in the delayed fracture test except that they were in the form of rotating-type cantilever tensile bars. specimen undergoes reduction in area with increasing load until break or fracture occurs. The extent of reduction in area of the specimen before fracture is the measure of the tensile strength of the metal under'test.

The specimen steel bars plated with cadmium by the cyanide bath process underwent a reduction in area of only 8% before fracture as against a reduction in area of about 45% for the specimen steel bars which had been plated with cadmium by the process of the invention and like bars of the steel which were unplated.

Since the principle of the invention herein described may be variously practiced without departing from the spirit or scope of the invention, it is to be understood that specific embodiments of the invention appearing in the above description are to be taken as illustrative and not in limitation except as may be required by the appended claims.

What is claimed is:

1. In the electrodeposition of cadmium, the improvement which comprises electrodepositing cadmium from a In this test, the

non-cyanide aqueous bath consisting essentially of a solution in water of a cadmium salt of a concentration of from about 0.5 to 2 molar to which triethanolamine has been added in amount such that the molar ratio of triethanolamine to cadmium. is from about 1:1 to 4:1 and the pH of the bath is in the range of from above 7 to about 10.

2. In the electrodepositing of cadmium, the improvement which comprises electrodepositing cadmium from a non-cyanide aqueous bath consisting essentially of a solution in water of a cadmium salt of a concentration of from about 0.5 to 2 molar to which triethanolamine has been added in amount such that the molar ratio of triethanolamine to cadimum is from about 1.25:1 to 1.55 :1 and the pH of the bath is in the range of from about 7.5 to 9.

3. A non-cyanide aqueous electroplating bath consist ing essentially of a water solution of a cadmium salt of a concentration of from about 0.5 to 2 molar to which has been added triethanolamine in amount such that the molar ratio of triethanolamine to cadmium is from about 1:1 to'4:1 and the pH of the bath is in the range of from above 7 to about 10.

' 4. A non-cyanide aqueous electroplating bath consisting essentially of a water solution of a cadmium salt of -a concentration of from about 0.5 to 2 molar to which has been added triethanolamine in amount such that the molar ratio of triethanolamine to cadmium is from about 1.25:1 to 1.55:1 and the pH of the bath is in the range of from about 7.5 to 9.

5. A process of preparing a non-cyanide aqueous cadmium electroplating bath consisting essentially of the steps of dissolving a cadmium salt in water to form a solution of from about 0.5 to 2 molar cadmium concentration and adding triethauolamine to said solution in an amount such that the ratio of triethanolamine to cadmium there-. in is from about 1:1 to 4:1 and the pH of the resulting solution is in the range of from above 7 to about 10.

References Cited in the file of this patent UNITED STATES PATENTS 2,377,228 Harford May 29, 1945 2,818,374 Certa et a]. Dec. 31, 1957 2,862,860 Vlannes et a1. Dec. 2, 1958 

1. IN THE ELECTRODEPOSITION OF CADMIUM, THE IMPROVEMENT WHICH COMPRISES ELECTRODEPOSITING CADMIUM FROM A NON-CYANIDE AQUEOUS BATH CONSISTING ESSENTIALLY OF A SOLUTION IN WATER OF A CADMIUM SALT OF A CONCENTRATION OF FROM ABOUT 0.5 TO 2 MOLAR TO WHICH TRIETHANOLAMINE HAS BEEN ADDED IN AMOUNT SUCH THAT THE MOLAR RATIO OF TRIETHANOLAMINE TO CADMIUM IS FROM ABOUT 1:1 TO 4:1 AND THE PH OF THE BATH IS IN THE RANGE OF FROM ABOVE 7 TO ABOUT
 10. 